Run taskgraph generation with Python 3

[ahal]

This fixes issue #4393.

We'd like to drop support for Python 2 in the taskgraph repo. To ensure the ability to address issues quickly, we'd like to first update consumers of it to Python 3.

[codecov-commenter]

Codecov Report

Merging #4394 (b0ed7d5) into main (94baebd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4394   +/-   ##
=======================================
  Coverage   76.00%   76.00%           
=======================================
  Files          46       46           
  Lines        4193     4193           
=======================================
  Hits         3187     3187           
  Misses       1006     1006           

Impacted Files	Coverage Δ
components/nimbus/src/enrollment.rs	97.57% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 94baebd...b0ed7d5. Read the comment docs.

[bhearsum]

I don't have I have permission to do an official review here, but LGTM

[ahal]

Latest force push was just rebasing on main (CI passed prior to that)

[skhamis]

Thanks for making these changes -- LGTM!

The pull request has been modified, dismissing previous reviews.

[ahal]

Thanks @skhamis ! I don't have permission to merge or apply labels, help merging would be appreciated. Also I rebased on top of latest main and apparently that invalidated your review (there weren't any code changes). Sorry about that.

[skhamis]

No problem @ahal! Yeah I think we have new merging rules that remove review after a rebase -- I'll go ahead and merge it now.